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I live in Washington state. I have a 20 amp circuit breaker on a circuit that drives two fan-forced heaters, connected through a single line voltage "dumb" thermostat, unknown rating. I would like to replace the "dumb" thermostat with a single wifi-connected smart thermostat.

The heaters look to be the same model and one of them has a nameplate shown below. Assuming the worst case of 2 x 1500 W, this should yield 3000 W / 240 V = 12.5 A.

Heater plate rating

Most line voltage smart thermostats I can find are rated at 16 amps. Can I use that on this circuit to control both heaters, or would I need a thermostat rated at 20 amps? 12.5 is less than 16 so it should work, but is it compliant with electrical codes?

From the below question it appears the answer is yes, but the second answer mentions "You can have multiple stats on one circuit. The code requires more than 1 you have 3 so this is up to code." I have only one thermostat - is that still ok? Using 15 amp thermostat on a 20 amp circuit

(I wanted to add a comment under that answer to ask this question but I don't have enough reputation points.)

  • You did the right thing by breaking this out into its own question, first off, as we're not a forum – ThreePhaseEel Oct 3 '17 at 22:12
  • Also, can you get us the nameplate ratings on the heaters in question? – ThreePhaseEel Oct 3 '17 at 22:12
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    Switches and controls on a circuit don't have to be rated for the circuit, they have to be rated for the load they are controlling. So, the question would be how much current do the two heaters draw. That is why TPE asked you for the nameplate ratings. – ArchonOSX Oct 3 '17 at 22:43
  • I agree with archonosx3 the switch has to be rated for the load not the size of the breaker.+ should be an answer. – Ed Beal Oct 4 '17 at 1:55
  • Why not add a relay for each heater, then the amperage of the heaters will not matter? – d.george Oct 4 '17 at 10:12
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First off, regarding fixed electric space heating equipment the National Electrical Code has this to say:

424.3 Branch Circuits.

(A) Branch-Circuit Requirements. Individual branch circuits shall be permitted to supply any volt-ampere or wattage rating of fixed electric space-heating equipment for which they are rated.

...

(B) Branch-Circuit Sizing. Fixed electric space-heating equipment and motors shall be considered continuous load.

Since the heater is considered a continuous load then the branch circuit serving it has to be sized 125% of the load. Consequently, a 20 amp circuit can only serve a 16 amp load.

The Code also says:

424.19(C) Unit Switch(es) as Disconnecting Means. A unit switch(es) with a marked “off” position that is part of a fixed heater and disconnects all ungrounded conductors shall be permitted as the disconnecting means required by this article where other means for disconnection are provided in the types of occupancies in 424.19(C)(1) through (C)(4).

...

(3) One-Family Dwellings. In one-family dwellings, the service disconnecting means shall be permitted to be the other disconnecting means.

So, an additional disconnect is required for the heater besides the main service disconnect.

Then the Code has this to say about thermostats:

424.20 Thermostatically Controlled Switching Devices.

(A) Serving as Both Controllers and Disconnecting Means. Thermostatically controlled switching devices and combination thermostats and manually controlled switches shall be permitted to serve as both controllers and disconnecting means, provided they meet all of the following conditions:

(1) Provided with a marked “off” position

(2) Directly open all ungrounded conductors when manually placed in the “off” position

(3) Designed so that the circuit cannot be energized automatically after the device has been manually placed in the “off” position

(4) Located as specified in 424.19

So, you can use a thermostat as both a controller and the additional disconnect.

Now, elsewhere in the Code it is allowed that a switch can be used as a disconnect provided it is sized for the load it serves.

404.14 Rating and Use of Switches. Switches shall be used within their ratings and as indicated in 404.14(A) through (F).

*Informational Note No. 1: For switches on signs and outline lighting, see 600.6.

*Informational Note No. 2: For switches controlling motors, see 430.83, 430.109, and 430.110.

(A) Alternating-Current General-Use Snap Switch. A form of general-use snap switch suitable only for use on ac circuits for controlling the following:

(1) Resistive and inductive loads not exceeding the ampere rating of the switch at the voltage applied

So, the branch circuit must be sized 125% of the load but the switch or thermostat in this case only has to be sized for the load. Consequently, this is why all the thermostats you have found are only rated for 16 amps since most branch circuits supplying these would be 20 amps and that would limit the load to 16 amps.

  • And thus this is also what would allow there to be 14 gauge wire on the thermostats then? The 15 amp rated thermostats generally have 14 gauge lead wires. – Scott B Jan 5 '18 at 6:37

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